How to display the checkout page under the iframe?

Hello , thank you for your further help on this —
How to display the checkout page under the iframe?

Under iframe, the checkout page shows “xxxx.com refused to connect”.
I checked the internet article that it is related to the X-Frame setting. How can I set this?

Note 1: Both the main site and target site are https

Note 2: I don’t know if the following is the old way, it doesn’t work after trying it. Or, my method is wrong, I use code snpit plugin to put the code into the website

https://gist.github.com/shivapoudel/8f40615c3f700a6274577d598718d1d9?fbclid=IwAR3OdJ2WuLQnSbzAeBABJ4tF0p214G3WeY4oJx40l9wEGIUgKVHOU6itMBs

Thank you

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